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I see many people growing Chlorella or Spirulina, types of microalgae, inside bottles... I see that they also insert CO2 into the bottles artificially... This question is basically comprised of two questions that will clarify this situation:

  1. Why do they use bottles or any type of bottle-like canister?
  2. Why do they insert CO2 to the bottle artificially in cases the bottle doesn't have a cork ? If the claim is that enough CO2 won't be able to get through the bottle neck than why growing the algae inside a bottle\aquarium in the first place, and not in a tub with full exposure to the air?
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  • $\begingroup$ Simple answer from justspirulina.org/spirulina-manual "Spirulina needs sunlight so it is preferable that the container in which it is grown be transparent. Empty, clear soft drink or water bottles (at least 1.5 litre) are a good option." $\endgroup$ – another 'Homo sapien' Mar 15 '16 at 15:59
  • $\begingroup$ @Benia I cannot find anything about "they insert CO2 to the bottle artificially" in the links provided by you. $\endgroup$ – another 'Homo sapien' Mar 15 '16 at 16:17
  • $\begingroup$ The thin tubes they insert, aren't these splitting CO2 into the water (as the bottle is sealed for CO2 entrance) ? $\endgroup$ – JohnDoea Mar 15 '16 at 17:16
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Really, the credit for this answer goes to another 'Homo sapien' from the Spirulina manual link in his comment.

Why do they use bottles or any type of bottle-like canister?

Likely, because they are clear. From the manual:

Spirulina needs sunlight so it is preferable that the container in which it is grown be transparent.

Plastic bottles are cheap, clear, and easy to come by, so they make the ideal material.

Why do they insert CO2 to the bottle artificially in cases the bottle doesn't have a cork?

I don't think the tubes in your picture are CO2 tubes. Rather, I'd bet they are bubble tubes (like in aquariums). These are necessary to keep the growth medium moving constantly. Again, from the manual:

Spirulina tends to gather at the top of the growing culture, where sunlight exposure is maximal. Due to this, Spirulina that cannot reach the top will not multiply and will ultimately die.

In order to maximize Spirulina exposure to sunlight, the water in which it is grown must be stirred ...

Another option is a pump, the simplest kind used for aquariums. (emphasis mine)

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  • $\begingroup$ Luigi, a small question, aren't at least some aquariums have CO2 tubes sometimes? I mean aren't some types of Fish or Algae hat does need extra CO2 into the water in some other cases? $\endgroup$ – JohnDoea Mar 16 '16 at 12:14
  • $\begingroup$ BTW please thumb up if you liked the question - If you would, I could thumb up your answer... $\endgroup$ – JohnDoea Mar 16 '16 at 12:14
  • $\begingroup$ @Benia I already have. And regarding the aquarium question, I'm not an expert on fish or algae, but I always thought the purpose of the tubes into aquariums was to move the water/make it more accessible for gas exchange, so that the oxygen/CO2 in the water could be closer to atmospheric levels (you don't want to suffocate your fish). While I'm sure there are specific organisms that prefer low CO2, I feel like most everything has evolved to grow best at atmospheric concentrations of O2/CO2 $\endgroup$ – Luigi Mar 16 '16 at 13:11
  • $\begingroup$ @Benia I do not think there would be any fish or algae that would have evolved to live by getting extra CO2 from artificial source. You see, as Luigi said, most organisms are adapted to live with natural atmospheric conditions (except, maybe, thermophiles which live near volcanic vents in deep sea surface). $\endgroup$ – another 'Homo sapien' Mar 16 '16 at 14:42
  • $\begingroup$ Well, according to what you said guys I know understand that this tubes insert some fresh air to the water and the excessive Oxygen or CO2 / any other air-material will just leave the water out through the bottle-neck (and open area if there is no cork) and thus will just split the algae in a more efficient way and avoid formation of a single light-hiding layer of algae in the outermost areas of the canister... I hope I understand you well guys, this is very helping for me and I am very grateful for you for your help !!! $\endgroup$ – JohnDoea Mar 16 '16 at 14:47

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